Controlling horn flies on pastured cattle

By Dave Boxler

University of Nebraska-Lincoln Research and Extension

Pasture fly season is approaching and now is the time to evaluate your horn fly management plan for the 2018 grazing season. First, re-evaluate last year’s plan. Did it provide adequate fly control? If yes, do you have a resistance management plan for the new fly season? If fly control was less than desired, now is the time to alter the plan and make necessary changes to reduce impact of horn flies on pastured cattle.

Large populations of horn flies on pastured cattle impose significant economic impacts in addition to affecting animal welfare. Horn flies on pastured cattle impact U.S. producers over $ 1 billion annually. Nebraska studies have shown calf weaning weights were 10 to 20 pounds higher when horn flies were controlled on mother cows. Studies conducted in the U.S. and Canada have shown horn flies can impact calf weaning weights from 4 to 15 percent. On a typical 500 cow ranch that could equate to an $8,000 to $16,000 increase in profit. Yearling cattle are also affected; other studies have shown yearling weights can be reduced by as much as 18 percent. Horn fly impact is measured by the economic injury level, which is defined as the lowest pest population density that will cause economic damage equal to the cost of treatment. The economic injury level for horn flies is 200 flies per animal, and fly control should be implemented if the EIL is exceeded. Horn fly numbers on untreated Nebraska cattle can easily exceed several thousand during late summer.

Horn flies are smaller than house flies, approximately 3/16 of an inch long, and are usually found on backs, sides, and poll area of cattle. During a warm summer afternoon they may be found on the belly region of cattle. Horn flies, both male and female flies acquire more than 30 blood meals per day and are almost always found on the animal. After mating the female fly will leave the animal to deposit eggs in fresh cattle manure and then return to the animal to resume feeding. Eggs hatch within one week and larvae feed and mature in manure, pupating in the soil beneath the manure pat. Newly emerged horn flies can travel several miles searching for a host. The entire life cycle can be completed in 10 to 20 days depending upon the weather. Multiple generations are produced during the summer with adult populations peaking in late summer.

Due to economic losses from horn flies, fly control is warranted. Many effective insecticide control methods are available to manage horn flies on pastured cattle. Selecting the most desirable control method for your operation will depend on efficacy, cost, convenience and herd management practices. Current horn fly control delivery methods are described below.

Dust bags—Dust bags deliver insecticides that are incorporated into a very fine dust that filters through the bottom of the bag when cattle contact the bag. The most effective way to use this delivery system is to locate bags so cattle must pass under the bags on their way to water, feed or mineral. This can be accomplished by fencing around water tanks and suspending the bags in the entrance/exit opening. Studies have shown dust bags placed in a free-choice arrangement will provided between 25 to 50 percent less control compared with forced-use dust bags. One dusting location with two bags is adequate for treating approximately 50 to 60 head of cattle. Dust bags should be hung at mid-shoulder level (of the cow), so cattle make maximum contact with the bags. Bags should be checked on a regular basis and recharged with insecticide dust as needed.

Back rubbers and oilers—As with dust bags, these devices are more effective when placed in a forced-use arrangement such as mineral stations or entrances to watering locations. Insecticides used with these devices should be mixed with No. 2 diesel fuel or mineral oil and should be recharged weekly. Do not use motor oil to dilute the insecticide as this will be harmful to cattle.

Pour-ons—Ready-to-use insecticide products applied in measured doses along the back line of animals. They provide fly reduction for several weeks, so re-application is required throughout the fly season depending on horn fly pressure.

Animal sprays—Insecticide sprays can be applied with low and high pressure sprayers or by mist blower sprayers. When using low and high pressure sprayers, cattle should be gathered and corralled to insure adequate spray coverage. Mist blower applications are made in the pasture where cattle are grazing, thereby reducing animal stress related to gathering and penning cattle. Animal sprays will provide 7 to 21 days of control and will need to be re-applied throughout the fly season.

Oral larvicides—Oral larvicides, or feed additives, Insect Growth Regulators, or IGRs, are insecticides that are incorporated into mineral blocks, tubs or loose mineral. These products prevent horn fly larvae in manure pats from becoming adults. Oral larvicides are effective when consumed in sufficient quantities throughout the fly season. Adult horn fly numbers may appear unaffected if cattle consuming feed additives are in close proximity to an untreated herd. An untreated herd may provide enough flies to keep fly numbers above the economic injury level for both treated and untreated cattle.

Insecticide ear tags and strips—Ear tags and strips have one or more insecticides embedded in a plastic matrix. Movement of the head or grooming of the animal slowly releases small amounts of insecticide over time that travels through the hair coat of the animal. In Nebraska, ear tags and strips should be applied during the last week of May or the first week of June to achieve maximum control through the fly season. Ear tags and strips applied too early may decline in efficacy while fly numbers are still high and result in economic loss. Adult animals should receive two tags or strips, tagging just the calf will not provide the desired level of horn fly control. All insecticide ear tags and strips should be removed at the end of each fly season to help manage fly resistance.

Compressed air application—The Vet Gun, a device similar to a paintball gun, applies an individual capsule of insecticide, VetCaps, to an animal and can provide horn fly control between 21 to 35 days. VetCaps can be used on all beef cattle weighing at least 600 pounds.

Regardless of your choice of application method, you need a Resistance Management Plan. Many horn fly populations in Nebraska exhibit a level of resistance to synthetic pyrethroid insecticides. The recommended practice to manage resistance is to alternate insecticide classes, and that applies to dusts, insecticide ear tags, animal sprays, pour-ons, feed-throughs (IGRs) and compressed air application devices.

Insecticides have been placed into numbered Insecticide Mode of Action groups based on how they work against insects. Continual use of products from a single group against a pest species can lead to reduce control (resistance to all products in the group). To improve fly control and minimize resistance, do not apply insecticides within the same group number repeatedly. Rotate between groups during the fly season.